1. Field of the Invention
The present invention relates to an antilock brake control system in a vehicle, including a front wheel braking liquid pressure regulating means capable of regulating the braking liquid pressure for a front wheel brake, a rear wheel braking liquid pressure regulating means capable of regulating the braking liquid pressure for a rear wheel brake, a front wheel speed detecting means for detecting a front wheel speed, a rear wheel speed detecting means for detecting a rear wheel speed, a front wheel-use vehicle speed calculating means for calculating a presumed vehicle speed based on the front wheel speed detected by the front wheel speed detecting means, a rear wheel-use vehicle speed calculating means for calculating a presumed vehicle speed based on the rear wheel speed detected by the rear wheel speed detecting means, a reference vehicle speed determining means for determining reference vehicle speeds based on the presumed vehicle speeds calculated respectively in both the vehicle speed calculating means, and a control quantity calculating means adapted to determine operation control quantities for the braking liquid pressure regulating means based on results of determination of wheel slip rates based on the reference vehicle speeds determined in the reference vehicle speed determining means and the wheel speeds detected respectively by the wheel speed detecting means, and to retard the pressure increasing timing in one of the front and rear wheel braking liquid pressure regulating means which corresponds to a wheel having a smaller load share, until the wheel speed of the wheel having the smaller load share is started to be reduced while approximating an actual vehicle speed after that wheel has restored to a level near the actual vehicle speed, when both of the front and rear wheel brakes are in an antilock brake-controlled state.
2. Description of the Related Art
An antilock brake control system is conventionally known, for example, from Japanese Patent Application Laid-open No.6-144193.
In the above-identified antilock brake control system, when both the front and rear wheel brakes are in the antilock brake-controlled state, the timing of increasing the braking pressure for the wheel having the smaller load share is intentionally retarded to cause such wheel to be coasted, in order that the wheel speed of the coasted wheel may follow or approximate the actual vehicle speed, for determining a presumed vehicle speed based on the wheel speed of the coasted wheel accurately to the utmost. In the above known system, however, the braking liquid pressure is reduced in retarding the pressure increasing timing and for this reason, there is a possibility that the braking force may be insufficient for the wheel having the smaller load share in traveling on a road having a relatively high friction coefficient. Especially in a motorcycle, the wheels are a front wheel and a rear wheel and hence, braking force shared by one of the wheels is large, as compared with a vehicle having a larger number of wheels such as a four-wheel vehicle. For this reason, if the braking force for one of the wheels becomes insufficient, the degree of the total braking force decreasing in the vehicle becomes larger.